In the course of manufacturing paper and similar products, including paper board and the like, it is well known to incorporate quantities of inorganic materials into the fibrous web in order to improve the quality of the resulting product. A number of inorganic materials have long been known to be effective for these purposes, such as titanium dioxide, which ca be incorporated into the paper in the form of anatase or rutile. Titanium dioxide, however, is among the most expensive materials which are so usable. Accordingly, in recent years, considerable efforts have been made to develop satisfactory replacements for the said titanium dioxide.
Among the materials which have thus found increasing acceptance as paper fillers are substantially anhydrous kaolin clays. Materials of this type are generally prepared by partially or fully calcining a crude kaolin clay, which may have been initially subjected to prior beneficiation steps in order to remove certain impurities, e.g. for the purpose of improving brightness in the ultimate product. It is important for an understanding of the present invention, to recognize that those skilled in the art of kaolin processing draw a sharp and fundamental distinction between uncalcined and calcined kaolins. With respect to terminology, it is noted that the prior art literature, including numerous of the prior art patents relating to the field of kaolin products and processing, often uses the term "hydrous" to refer to a kaolin which has not been subjected to calcination--more specifically, which has not been heated to temperatures above about 450.degree. C., which temperatures serve to alter the basic crystal structure of kaolin. These so-called "hydrous" clays may have been produced from crude kaolins, which have been subjected to beneficiation, as, for example, to froth flotation, to magnetic separation, to mechanical delamination, grinding, or similar comminution, but not to the mentioned heating as would impair the crystal structure.
In an accurate technical sense, the description of these materials as "hydrous" is, however, incorrect. More specifically, there is no molecular water actually present in the kaolinite structure. Thus, although the composition can be (and often is) arbitrarily written in the form 2H.sub.2 O.Al.sub.2 O.sub.3.2SiO.sub.2, it is now well-known that kaolinite is an aluminum hydroxide silicate of approximate composition Al.sub.2 (OH).sub.4 Si.sub.2 O.sub.5 (which equates to the hydrated formula just cited). Once the kaolin is subjected to calcination, which, for the purposes of this specification means being subjected to heating of 450.degree. C. or higher for a period which eliminates the hydroxyl groups, the crystalline structure of the kaolinite is destroyed. As used in this specification, the term "calcined kaolin" shall refer to such a kaolin. Preferably the calcined kaolin has been heated above the 980.degree. C. exotherm, and therefore is "fully calcined", as opposed to having been rendered merely a "metakaolin". Reference may be had in the foregoing connection to Proctor. U.S. Pat. Nos. 3,014,836 and to Fanselow et al, 3,586,823, which disclosures are representative of portions of the prior art pertinent to fully calcined kaolins.
A calcined product having characteristics generally superior to previously available such calcined kaolin pigments, is the ALPHATEX.RTM. product of E.C.C. America Inc., assignee of the present application. This product again is a substantially anhydrous white kaolin clay pigment, which has unusual efficacy as a filler in paper sheets and similar paper products. The pigment also has application as a coating pigment for paper, and as a pigment in paints and other filled systems. It generally consists of aggregates of anhydrous kaolin clay particles, and exhibits exceptionally high light-scatter and opacifying characteristics when incorporated as a filler in paper.
ALPHATEX.RTM. is further described in U.S. Pat. No. 4,381,948 to A.D. McConnell et al, as being an anhydrous white kaolin clay pigment having high light scattering when incorporated as a filler in paper, the pigment consisting of porous aggregates from sub-micron sized kaolin clay platelets obtained by classification of a dispersed kaolin clay to a 100% less than one micron ESD fraction, the aggregates having an average specific gravity in the range of 0.5 to 0.6 and a mean internal pore size of less than 0.55 microns. The size distribution of the aggregates is such that no more than 5% by weight thereof are greater than 10 microns ESD, at least 75% are of less than 2 microns ESD, and not more than 15% by weight are of less than 1 micron ESD. The pigment has a Valley abrasion value below 30 mg., and a G.E. brightness of at least 93.
Calcined kaolin clay products such as ALPHATEX.RTM. are normally pulverized in a high energy impact mill and air-classified after calcination for the purpose of removing +325 mesh residue (to conform to specification for intended use in paper), or in order to remove larger abrasive particles. In order to increase the bulk density and the wet-out characteristics of such products--thereby to facilitate bulk handling--it is disclosed in U.S. Pat. Nos. 4,593,860 and 4,693,427, that the anhydrous calcined kaolin clay powder can also be dry milled in a media mill, using work inputs of from about 5 to about 40 hp-hr/ton of dry clay.
Calcined kaolin clay products are often handled, shipped and/or utilized (e.g. when coating papers) as aqueous slurries, i.e., the calcined kaolin clay is slurried in water to form a slurry, e.g., of approximately 40% to 60% solids content, more or less. It is desired that the slurry not be unduly viscous since the more viscous the product generally the more difficult it is to handle and to use; but in the past this has proved to be a most vexatious problem, particularly wher the solids content of the slurry begins to exceed about 50%.
A large number of prior art patents are concerned with the general problem of reducing clay viscosity, such as Duke 3,326,705; Turner 4,334,985 and 4,468,317; Maynard 3,808,021 and 3,857,781; and Abercrombie. Jr. 4,144,084 and 4,144,085. These teachings, however, are applicable to hydrous clays, and commonly involve chemical control by additives or elimination of a species tending to cause viscosity problems. The prior art bearing upon viscosity control in calcined clay slurries is more limited, but often involves some type of milling or comminution of the calcined material.
According to Cecil. U.S. Pat. No. 3,754,712, for example, a fluid suspension or slurry of calcined kaolin clay is wet milled in the presence of grinding media, with incremental additions of calcined kaolin clay being added and wet milling continued until the viscosity of the slurry is reduced. Also of interest in this connection are Horzeoa et U.S. Pat. No. 4,118,246; and Hamill et al 4,118,245. The latter mentions the use of energy inputs of 200 hp-hr/ton; but both of these references concern wet milling, a procedure which is very difficult from an engineering viewpoint, requiring very complex and sophisticated controls. While more applicable to chemical stabilization of calcined clay slurries, reference may also be had to Tapper 3,846,147; and Eggers 4,017,324.
While the foregoing teachings are of value, it is desirable to be able to reduce the viscosity of slurries of calcined clay, without adding special chemical agents to the slurry, and without the use of the prolonged and complex processing inherent in the wet grinding methods of teachings such as Cecil, Hamill et al, Horzepa et al, etc.
Dry grinding of calcined kaolins, as mentioned, is taught in the present assignee's U.S. Pat.s Nos. 4,593,860 and 4,693,427. Also a broad teaching of frictionally working or milling a calcined kaolin to decrease its clay-water viscosity and adhesive demand, appears in Proctor. Jr., U.S. Pat. No. 3,014,836, where, however, wet milling is disclosed as the preferred and exemplified technique. None of this prior art, however, has disclosed how dry milling may be effectively utilized to produce a superior coating pigment.
More generally, calcined kaolin clay pigments such as the above described ALPHATEX.RTM., have been of increasing interest to paper manufacturers for use in paper coating. In common paper coating applications, the pigment properties sought often include high gloss, good printability, good opacity and high brightness. Where calcined kaolins are used, manufacturers would prefer to utilize a paper coating composition (the "coating color") which desirably incorporates from 15 to 20 parts pe hundred (by weight) of the calcined kaolin--this in place of more expensive pigments used in the past, such as titanium dioxide. Overall, the coating color in order to be most effective should include about 60 to 63% by weight of total solids. With prior art calcined pigments, all of these requirements could not be readily achieved. In particular, in order to achieve the mentioned parts of calcined kaolin in the 60 to 63% solids coating color, it is necessary to be able to use or handle the kaolin pigment at its equivalent of 60% solids; so that the overall requirement is that the kaolin pigment has good rheological properties as a 60% solids aqueous slurry--and also provide fully acceptable opacity, gloss, brightness, printability, etc. when coated. The prior art such pigments, while capable of being formed into such high solids slurries, tended to provide under such conditions very high dilitancy, and thus poor rheological properties. In an effort to allow the prior art calcined pigments to be used, lower solids can be employed, but this tends to leave excess water in the coating color--with resultant running during coating.
A further property of the prior art calcined pigments that has diminished the value of same as coating pigments, arises from the very porous highly aggregated nature of the component structures of same. While these properties foster high opacity, i.e., good light scattering, they have a detrimental effect on gloss when the pigment is used in coating.